使用没有显式强制转换的常量调用未定义行为？

Question

Upon invoking a function,or returning a value from a function,that expects a value of type T ,does using a constant literal without explicit cast invoke undefined behavior?

For example,we have a function who's prototype is long foo(unsigned long x); Invocation: foo(4); //does this invoke UB? foo(4); //does this invoke UB?

long foo(unsigned long x) { x += 10; return 10; } // does this invoke UB ?

should we write foo((unsigned long)4) and return (long)10 ??

Answer 1

No, it is all well-defined.

There exists an implicit conversion rule between the two types, so the int is simply converted to an unsigned long and the program works as expected.

Answer 2

Type of literal 4 is int . (In C section 6.4.4.1 Integer constants, similar section available in C++ also)

Implicit conversion from int to unsigned long is well defined in both C and C++. (In C section 6.3.3.1)

should we write foo((unsigned long)4) and return (long)10?

Both of your example are well defined, so this conversion though acceptable is superfluous.

Answer 3

Consider this C code:

// foo.c
int foo(unsigned long x) { }

and

// main.c
int foo();

int main()
{
     foo(4);    // UB
     foo((unsigned long)4);   // OK
}

The foo(4) call is UB because when you are calling a function with no prototype in scope, you must manually make sure the parameters match. The default argument promotions occur but that's all.

Of course, writing the cast is a bad solution from the point of view of writing robust code. The better solution would be to write a prototype:

int foo(unsigned long);

in a header file which is included from both the .c files.

---

The return 10; case can never be UB because the function's true return type is known to the compiler when it is compiling the code inside the function body.

Answer 4

No, it wouldn't make sense, since such argument is passed by value in both C and C++:

long foo(unsigned long x);

You may think of it technicaly as x parameter is a local automatic variable defined inside foo and assigned with the value of passed argument:

unsigned long x = 4;

If type of argument does not match with parameter, then compiler tries with implicit conversion. For instance, an argument of type double is silenty converted into of type unsigned long , even it means loss of information (you might get a compiler warning though).

You may however get into trouble, when you mark type of x parameter as reference (C++ only):

long foo(unsigned long& x);

Here, the compiler would not allow you to call it as foo(4) , because you are now passing by reference, and 4 cannot be modified as such. You may however pass it if the parameter is marked with const qualifier:

long foo(const unsigned long& x);